Automotive air conditioning compressor housings are typically multi piece canisters which must maintain a gas tight, pressurized interior against leakage. A cylindrical body is closed by at least one end cap, and several elongated bolts hold the end cap in place. A closed or "blind" threaded hole receives the threaded end of the bolt, which need not be sealed. The head of the bolt covers an exterior opening through the housing, and must be sealed gas tight. The conventional sealing system is portrayed in the first four Figures of the drawings, in which:
FIG. 1 is a cross section of a compressor housing; PA1 FIG. 2 is an enlargement of the exterior opening of a bolt hole with the bolt head and a sealing washer pulled axially away from the sealed position; PA1 FIG. 3 is a view like FIG. 2, but showing the bolt head and sealing washer in place; PA1 FIG. 4 is an enlarged view looking down into the bolt hole after the bolt and washer have been removed.
Referring first to FIGS. 1 and 2, a typical compressor housing, indicated generally at 10, consists of multiple parts closed by an end cap 12. The cap 12 and other parts are held together by several bolts, one of which is indicated generally at 14. The housing parts, including the end cap 12, are usually made of cast aluminum alloy, for weight reduction and ease of manufacture. The bolts 14 are steel, for strength. Each bolt passes through a coaxial bolt hole 16 (actually a series of aligned holes) with some radial clearance, to allow for ease of insertion, and is threaded at the end into a blind, closed hole 18 that needs no sealing. At the opposite end, however, the bolt hole 16 opens through a flat, annular sealing surface 20 that is machined down into the outer surface of cap 12, creating a sharp circular edge of equal diameter 22. Sealing surface 20 is machined just to assure a suitable flat surface, since the outer surface of cap 12 may be curved or pebbled. A gas tight seal must somehow be provided continuously around the bolt hole edge 22 to prevent pressure loss from housing 10. Traditionally, a seal has been provided, indirectly, between the sealing surface 20 and a flat, annular undersurface or "pad" 24 of a hexagonal bolt head 26. The bolt head pad 24 stands proud of the bolt head 26 in order to prevent interference with the bolt head 26 and the inset sealing surface 20. The bolt head pad 24 merges into the main cylindrical bolt body 28 across a small rounded "corner" 30, which comprises approximately a quarter circle in cross section. The corner 30 serves no purpose other than the typical avoidance of stress concentration at what would otherwise be a sharp corner, and has no role to play in sealing. Likewise, the bolt hole edge 22 plays no part in sealing. Sealing is entirely a function of the tight capture of an annular sealing washer 32 between the axially opposed bolt head pad 24 and the sealing surface 20, of an annular sealing washer 32.
Referring next to FIGS. 2 through 3, sealing washer 32 has an inner diameter that surrounds the bolt hole edge 22, but is large enough to clear the rounded corner 30, and thereby make contact only with the flat surface of the pad 24. The thickness of the washer 32 prevents the bolt corner 30 from contacting the bolt hole edge 22, or any other part of the sealing surface 20. Therefore, the upper and lower surfaces of the washer 32 make flat to flat contact only between the bolt head pad 24 and the sealing surface 20 respectively. Washer 32 is typically made up of material softer than bolt 12, such as copper or plated copper, and, ideally, also softer than the aluminum material of cap 12. Theoretically, washer 32, if compressed sufficiently, is supposed to yield and fill any voids or irregularities in the surfaces of bolt head pad 24 and end cap sealing surface 20, providing a gas tight seal. In actual practice, washers like 32 have been found to turn with the bolt head 26, rubbing, scratching and galling on the sealing surface 2D. The result of this action can be seen, once the bolt 14 and washer 32 have been removed, as an irregular, shiny circle on the sealing surface 20 surrounding edge 22. Now, so long as this circle covers 360 degree, it is evidence of a complete seal. And, in practice, satisfactory sealing is obtained, on a one time basis, at least, by a very careful control of the pressure with which the bolt 14 is torqued down against washer 32. However, because of the scraping action, and sometimes even deformation of the washer 32, it is difficult to make the seal repeatable. That is, it may be impossible to remove and replace the bolt 14 with the same washer 32. Also, it is always preferable that assembly operations of any sort be less dependent upon holding any parameter, such as torque or pressure, within very close limits.